1. Field of the Invention
This invention relates to a process for preparing nitrogen trifluoride (NF.sub.3). Nitrogen trifluoride normally is a colorless gas having a boiling point of about -129.degree. C and a melting point of about -208.degree. C. It is useful as a fluorine source in the preparation of fluorocarbons, especially fluorolefins or as an oxidizer for high energy fuels. It has advantages over elemental fluorine as a fluorine source in that it is relatively inert at low temperatures, e.g. 70.degree. F, whereas fluorine is not. Further, it can be compressed to high pressures, e.g. 1,000 psig for shipment.
2. Description of the Prior Art
U.S. Pat. No. 3,304,248 discloses a process for forming nitrogen trifluoride by passing gaseous nitrogen through a plasma arc at a temperature in excess of 1,000.degree. C and introducing gaseous elemental fluorine into the post arc region as near the anode as possible. The molar ratio of nitrogen to fluorine during such process is maintained in excess of 0.4:1.
U.S. Pat. No. 3,055,817 discloses a process for producing nitrogen trifluoride by reacting hydrazoic acid gas (HN.sub.3) with oxygen difluoride (OF.sub.2) in the presence of actinic (ultraviolet) radiation.
U.S. Pat. No. 4,001,380 discloses an improved process for preparing nitrogen trifluoride from FN.sub.3 by reaction with nitrosyl fluoride or chlorine trifluoride at a temperature above the boiling point of fluorine azide but below 100.degree. C.
U.S. Pat. No. 3,043,662 discloses a process for preparing nitrogen trifluoride by reacting carbon tetrafluoride or carbonyl fluoride with binary oxides of nitrogen, e.g. nitrous, nitric and nitrogen oxide. The reaction is carried out by passing the gases through an electric arc at a temperature of 2,000.degree. to 4,000.degree. C for a period of 0.001 to about 2 seconds.
The article "Fluorination of Ammonia" 82 J. AM. CHEM. SOC. 5301-4 (1960) discloses the synthesis of nitrogen trifluoride by the direct vapor phase fluorination of ammonia using molar ratios of ammonia to fluorine of from about 1.1-2:1. The reaction temperatures for the vapor phase process were not disclosed, but the authors reported yields of from about 10 to 25% NF.sub.3 based on fluorine. Some of the problems associated with this particular process are (a) the competing decomposition reaction of nitrogen trifluoride to nitrogen and hydrogen fluoride, (b) heat removal and temperature control, and (c) the generation of ammonium fluoride which can cause plugging in the recovery system downstream.
The fluorination of ammonium fluoride at a temperature of about 20.degree.-70.degree. C, optionally in the presence of sodium chloride, is shown in 13 RUSSIAN JOURNAL OF INORGANIC CHEMISTRY 1618-1619 (1968).
U.S. Pat. Nos. 3,235,474 and 3,356,454 disclose a process for preparing nitrogen trifluoride, with some by-product dinitrogen difluoride. These processes contemplate the electrolysis of ammonium acid fluoride at a temperature above its melting temperature, along with simultaneous injection of hydrogen fluoride. The basic idea in these patents is to avoid the generation of an explosive atmosphere which results because of by-product hydrogen being produced at the anode, and this is done by maintaining the nitrogen fluoride at a concentration lower than 9.4% or greater than 95%.